Carbon dioxide is often found as an impurity in natural gas and landfill gas, where methane is the major component. The presence of CO2 reduces the energy content of natural gas and can lead to pipeline corrosion (references 1-7). If natural gas meets established purity specifications, it is designated “pipeline quality methane,” which increases its commercial value. To meet pipeline requirements, natural gas must comply with strict CO2 concentration limits, as low as 2% (references 1,2,5).
For the separation of CO2 from natural gas, several technologies, such as absorption, cryogenic distillation, membrane separation, and adsorption, have been used. Among these technologies, adsorption-based methods such as pressure swing adsorption (PSA) are promising because of their simple and easy control, low operating and capital investment costs, and superior energy efficiency (references 1, 8, 9). In particular, adsorption is advantageous for the case of medium- and small-size processes (references 1, 9). After the suggestion by Sircar in the late 1980 (reference 10), many studies have been performed on PSA processes for the separation and purification of CO2 from gaseous streams containing CH4 (references 1, 3, 4, 11).
A key step in the design of PSA processes for the separation and purification of CO2 is the selection of a highly selective adsorbent with a high CO2 capacity (references 1, 8, 9, 11). Most studies of CO2/CH4 separation have focused on zeolites (references 1, 2, 6, 12, 13) and carbon based adsorbents (references 3, 4, 8, 13, 14).
Recently, metal-organic frameworks (MOFs) have been recognized as a new family of porous materials that have potential applications in separations, sensing, gas storage, and catalysis (references 15-17). MOFs consist of metal or metal-oxide corners connected by organic linkers. They are synthesized in a self-assembly process from these well-defined building blocks and have high porosity and well-defined pore sizes. The synthetic strategy opens up the possibility to systematically vary pore size and chemical functionality in the search for an optimal adsorbent. For separations, an additional advantage is that MOFs can be regenerated under milder conditions than most zeolites, which require considerable heating and the associated high costs (reference 18).
To date, most studies of adsorption in MOFs have focused on single-component gases, and little is known about mixture behavior even though understanding multicomponent adsorption equilibrium is essential for designing adsorption-based separation processes. For CO2/CH4 mixtures in MOFs, all of the published work to date has come from molecular simulation. Yang and Zhong used grand canonical Monte Carlo (GCMC) to simulate mixtures of CO2 and CH4 in Cu—BTC and MOF-5 (references 19-20). At 1 bar and 298 K, they predicted that Cu—BTC has a selectivity of about 6 for CO2 over CH4, and MOF-5 has a selectivity of about 2, independent of gas-phase composition. Babarao used GCMC simulations to compare CO2/CH4 mixtures in MOF-5, the zeolite silicalite, and C168 schwarzite (reference 13). They found that MOF-5 has a larger storage capacity, but the selectively is similar in all three materials. Both groups report that the simulated mixture behavior matches well with the behavior calculated from single-component isotherms using the ideal adsorbed solution theory (IAST) (references 13, 19, 21).